The Shocking Truth About What You'll Actually Pay
On a $350,000 30-year mortgage at 7%, your total repayment is $837,492 — meaning you pay $487,492 in interest alone, nearly 1.4 times the original loan. Most borrowers never do this math. Understanding the formula behind the payment gives you the power to compare, negotiate, and save tens of thousands of dollars.
The Core Formula
Monthly mortgage payment (P&I) is calculated using the fixed-payment loan formula:
M = P × [r(1+r)^n] / [(1+r)^n − 1]
Where:
- M = monthly payment
- P = principal loan amount
- r = monthly interest rate (annual rate ÷ 12)
- n = total number of payments (years × 12)
Step-by-Step Worked Example
Loan: $300,000 | Rate: 6.5% annual | Term: 30 years
- Convert rate to monthly: 6.5% ÷ 12 = 0.5417% = 0.005417
- Calculate n: 30 × 12 = 360 payments
- Calculate (1+r)^n: (1.005417)^360 = 6.8485
- Numerator: 0.005417 × 6.8485 = 0.037104
- Denominator: 6.8485 − 1 = 5.8485
- Factor: 0.037104 ÷ 5.8485 = 0.006343
- Monthly payment: $300,000 × 0.006343 = $1,902.85
Breaking Down Month 1 vs. Month 360
Each month, interest is calculated on the remaining balance. Here's month 1:
- Interest portion: $300,000 × 0.005417 = $1,625.10
- Principal portion: $1,902.85 − $1,625.10 = $277.75
- New balance: $300,000 − $277.75 = $299,722.25
And month 360 (final payment):
- Remaining balance: ~$1,892
- Interest portion: ~$10.25
- Principal portion: ~$1,892.60
This is why early payments feel like they make no dent — 85% goes to interest in year one. By year 25, the split reverses and you're mostly paying principal.
How Rate Changes Affect Your Payment
| Rate | Monthly P&I (on $300k/30yr) | Total Interest Paid |
|---|---|---|
| 5.5% | $1,703 | $313,080 |
| 6.5% | $1,896 | $382,560 |
| 7.5% | $2,098 | $455,280 |
| 8.5% | $2,307 | $530,520 |
Each 1% rate increase on a $300k mortgage adds roughly $150–$200/month and $55,000–$75,000 in total interest. This is why rate shopping — even for a quarter point — matters enormously.
The Impact of Down Payment on Your Payment
Increasing your down payment does two things: it shrinks the loan principal AND eliminates PMI once you cross 20%. On a $375,000 home:
- 5% down ($18,750): Loan = $356,250 + ~$180/mo PMI = $2,550/mo total
- 10% down ($37,500): Loan = $337,500 + ~$135/mo PMI = $2,390/mo total
- 20% down ($75,000): Loan = $300,000, no PMI = $2,100/mo total
The jump from 10% to 20% down saves $290/month — that's $3,480/year.
Adjustable Rate vs. Fixed: The Payment Shock Math
A 5/1 ARM might start at 5.5% (saving $200/month vs. a 7% fixed) but can adjust up to 2% per year after year 5. If rates rise to 9.5% by year 7, that same $300k mortgage shoots to $2,520/month — a $620/month jump. The formula still applies; the variable is just the rate, which changes annually after the fixed period ends.
Extra Payments: The Math Behind the Savings
Adding $200/month to principal on the $300k/6.5%/30yr example:
- Loan pays off in approximately 24 years (saves 6 years)
- Total interest saved: approximately $68,000
- Every extra dollar in the early years saves roughly $2–3 in future interest
Use the CalcPeek mortgage calculator to model any loan scenario, compare 15 vs. 30 year options, and see exactly how extra payments change your payoff timeline.
Bottom Line
The mortgage payment formula is deterministic — your lender uses the exact same math. Knowing it lets you verify every quote, understand what you're actually agreeing to, and make strategic decisions about down payments, rate buydowns, and extra principal payments that can save you six figures over the life of a loan.